The PC crystallization process is measured by the photoresistor, by which the reciprocal relationship is obtained between the illumination of transmission light and the resistance. In the beginning of annealing the change of resistance is insignificant when the crystallization is still at the induction stage. Then the increasing number and size of spherulites scatter the incoming light and the resistance increases quickly. Finally the concentration of amorphous PC decreases and the increasing rate of resistance slows down. The X-ray pattern shows an ordered PC structure of 2 theta = 11 degrees in the induction period, then converts to amorphous (broad peak at 20 degrees) and spherulite (17.2 degrees) structure. The dissolving heat of PC spherulite, analyzed by differential scanning calorimetry, shows a linear relationship to the resistance. Thus the relative crystallinity can be obtained by measuring the resistance of the photoresistor from both primary and secondary crystallization. The experimental results show good correlation to the Avrami model and the highest crystallization rate is obtained at 80 degrees C. The thermodynamic factor controls the crystallization rate at higher annealing temperature while the kinetic factor dominates the crystallization rate at lower annealing temperatures.